1. Field of the Invention
The present invention relates generally to inspection of semiconductor devices being fabricated, and more particularly to a Fourier filter for detecting defects in repetitive features on semiconductor wafers.
2. Description of the Background Art
Known methods exist to make inspection instruments for detecting defects on semiconductor wafers. These inspection instruments normally detect defects using some means to filter out the desired patterning on the semi-conductor wafers. There are optical filter implementations in the prior art, such as a liquid crystal display (LCD)-based filter and a photographic-based filter, that have been used in inspection instruments. U.S. Pat. No. 5,537,669 to Evans et al. describes another method and apparatus for the inspection of random or repeating patterns using a hybrid of spatial domains and frequency domain, i.e., "Fourier-like" transforms. These prior art filtering implementations have various disadvantages such as optical distortion, poor contrast, and process complexity.
The basis for defect detection takes advantage of the repetitive nature of the patterning on wafers to compare images of nominally identical features, with differences being identified as defects. In general, wafer patterns are repetitive on two scales: on the coarse scale, a single die or functional device is repeated many times across the wafer; these dies will later be separated and packaged separately. Dice are typically between 4 and 25 mm on a side, and rectangular in shape. On the fine scale, many devices have repetitive geometries within a die, such as an array of memory cells, where the repeating pattern has a period of several microns (.mu.m.), typically 2 to 20 .mu.m. This array area is where a Fourier filter can be of particular utility.
When an array on a wafer die is illuminated with coherent, collimated light to detect defects, the array serves as a reflective diffraction grating that produces diffraction spots which can be seen clearly in the pupil (Fourier) plane of an imaging system.
FIG. 1 illustrates a conventional Fourier filter mechanism 100 for blocking the diffraction pattern created by repetitive patterning (not shown) on a wafer 110. A laser light beam 101 incident on wafer 110 produces diffraction orders 102 and a specular reflection 103. An objective lens 104 redirects diffracted light rays through an LCD-based or photographic-based filter positioned in Fourier plane 105 and through a second lens 106 to focus an image on an image sensor 107.
FIG. 2A shows a repeating array located at the wafer image plane and FIG. 2B shows the parallel and linear array of Fourier spots 200 produced by a collimated laser beam diffracted from the array.
By placing in the Fourier plane a blocking mechanism that occludes the diffraction spots, while leaving the rest of the Fourier plane unobstructed, it is possible to remove the repeating content without removing the non-repeating content (the defects) from the array area of the image. Removing the array repeating content is advantageous for two reasons:
First, the action of defining and comparing "identical" features on the wafer is inherently error-prone because the precise extent of the repeating pattern must be defined, and images must be precisely acquired and subtracted. If the repeating content is removed optically, the array area appears as an unpatterned wafer. The definition, image acquisition and comparison of adjacent repeating cells create optical clutter that is avoided when the repeating pattern is removed by the use of a Fourier filter.
Second, in order to perform the comparison described above, an accurate image of the repeating pattern is required, which means that the gain of the imaging system cannot be increased beyond the point where the image approaches saturation of the detector. This limits the size of defects that can be seen. If the imaging system gain were further increased, smaller defects might become detectable, but because the repeating pattern would be in saturation, the array features would be poorly resolved and comparison of adjacent repeating cells would result in large amounts of noise that would swamp the signal from the smaller defects. If the repeating pattern is removed optically using a Fourier filter, the system gain can be freely increased because the repeating image is removed before it reaches the detector. The Fourier filter therefore allows much larger gains to be used in array areas, and consequently much better defect sensitivity.
A practical Fourier filter must allow the quick detection of defects on semiconductor wafers, which means that the filter should not be susceptible to having the filter output affected by the normal vibrations present in a manufacturing environment. Therefore, what is needed is a low-cost Fourier filter mechanism in which the damping of vibrations requires a minimum of time and the damping means does not interfere with the utility of the Fourier filter itself.